An example of marking a paper web by a laser beam with the aid of an image-bearing mask is disclosed in European patent specification EP-A 0279505. In this specification, it is shown how a laser beam may be brought to a line focus, the line extending the required width in the direction transverse to the scan direction. In the embodiments disclosed in that specification the mask is scanned past the fixed line provided by the beam.
Copending European patent application 89301486.0 filed Feb. 16, 1989 and published under the number EP-A 0329438 on Aug. 23, 1989 discloses another arrangement using a laser beam for marking a web with the aid of a mask or stencil. In this case an unfocussed, but collimated, laser beam is employed. The beam has a cross-section whose diameter satisfies the width requirement of the image transverse to the direction of scan.
The present invention has arisen out of work done in investigating the uniformity of marking obtained when a laser beam is scanned relative to the surface of a material, and more particularly the uniformity in the direction normal to the scan direction. More generally this has led to the consideration of obtaining a uniform treatment of the surface or surface zone of a material by scanning a laser beam thereover in which energy is applied over a significant width transverse to the scan direction.
At the same time as seeking to achieve this uniformity, it is desirable to achieve an efficient utilisation of the beam and to avoid having to use a laser of unduly excessive power. Take for example, a beam having a simple circularly symmetrical, Gaussian distribution, i.e. a TEM.sub.00 mode beam. It has a high central peak of irradiance rapidly dropping to low irradiance levels away from the centre. It will be readily appreciated that the strip irradiated by such a beam on sweeping a surface will receive a far greater total exposure where the peak sweeps than at the lateral margins. More uniform exposure can be achieved by truncating the beam--that is masking off the lateral portions--to concentrate on the central area. This reduces the width of strip treated as well as wasting a substantial portion of the beam power. If the beam diameter is increased to make up for the lost width, then a higher power laser is required to achieve the same irradiance in the active portion of the beam.
We shall describe hereinafter how by proper selection of the operating mode of the laser, and specifically by use of a ring mode laser, it is possible to achieve a substantially uniform delivery of energy across a major proportion of the scanned width of an untruncated beam. That is to say that the efficiency of utilisation of the beam is good if the beam is truncated to the appropriate proportion; or put another way for a given width scanned, the required power of the laser may be kept to a minimum for the surface treatment required. More particularly, it has now been discovered that to achieve these desirable results the rate of scan of the beam should be such as to result in an integrated effect on the material treated to which the name "exposure integration" is given.